1. Field of the Invention
The invention relates to a peak-hold circuit for holding a peak value of an input signal and a signal strength indicator using the peak-hold circuit, and specifically, relates to a peak-hold circuit, whose operation is an independent from temperatures, power supplies and processes, having a relatively less complicated circuit configuration, and a signal strength indicator using such a peak-hold circuit.
2. Description of the Related Art
In recent years, a walkie-talkie is widely used by the advance of mobile communication technology. A RSSI (Received-Signal Strength Indicator) is used in these devices. Although the general purpose of the RSSI is to measure and display the strength (electric power) of an input signal, the result based on the signal strength is not only fed back to an amplifier circuit of a receiver to be used for automatic gain control, but also requires the transmitter to send another signal based on the strength of the current input signal. Moreover, when the device having a RSSI is used as transmitter, RSSI may estimate its own signal strength and may amplify it to be a signal having a desired strength. Thus, RSSI serves as important functional block used inside the transmitter-receiver of wireless system. This conventional RSSI is disclosed in the reference JP2003-163556
FIG. 2 shows such a conventional RSSI 100. The RSSI 100 outputs a received-signal strength RSS based on the output of each saturated amplifier 101˜104, which is connected in four stages. Each saturated amplifier 101˜104 has two gain control terminals VC1 and VC2. At the gain control terminal VC1, a bias signal generated in a constant gm bias generator 151 is applied in order to amplify in a constant rate by the saturated amplifier 101˜104 in each stage, regardless of temperature. On the other hand, at the gain control terminal VC2 of the saturated amplifier 101, the output from the saturated amplifier 101 is applied through a rectifier 111, a low pass filter 121, and a bias generator 131 for controlling amplitude. Other gain control terminal VC2 of other saturated amplifiers 102˜104 receives a signal similarly through similar components, as well as the terminal VC2 of the amplifier 101. Each bias generator 131˜134 generates a bias signal for controlling the amplitude of the signal from the each saturated amplifier 101˜104. In other words, the signal from the each saturated amplifier 101˜104 is controlled by the bias signal from the bias generator 131˜134 not to exceed the determined value of the amplitude. The signals outputted from the low pass filters 121˜124 are added by an adder circuit 141, and the result of adding is outputted as the comprehensive received-signal strength RSS.
The conventional RSSI 100 as shown in FIG. 2 includes the saturated amplifiers 101˜104 as a gain commensuration means and the bias generator 131˜134 as an adjuster means for limiting the saturation amplitude in order to amend the received-signal strength RSS, which is affected by the temperatures, power supplies and processes, on one-chip formation, without using some external components. However, as shown in FIG. 2 or the reference JP2003-163556, each of which discloses an example of a specific circuit, the conventional RSSI 100 requires a lot of circuit components.